+1 for Genghis' advice:
Basically, do it actively. Reading the book will not get enough into your head for the vast majority of normal human beings.
Make your own notes from the books, abbreviate them, set and solve problems: basically do "stuff" rather than just read "stuff".
Never been able to make any subject stick by merely reading books or scripts. Always used my own extracts (mostly handwritten despite my abysmally scratchy writing, sticks slightly better than typewritten notes), whether at university or for the PPL.
Cannot comment on apps, my mobile phone is almost as old as some planes in the club hangar 